The present invention relates to a feedwater controller, a nuclear power plant equipped therewith and a method for controlling feedwater.
As a controlling method of a feedwater flow rate to a nuclear reactor, a rotation speed control of a reactor feedwater pump driven by a turbine and feedwater flow rate adjustment valve control of a reactor feedwater pump driven by an electric motor are known.
On the other hand, a feedwater apparatus using a feedwater pump driven by an electric motor whose rotation speed is variable, with an electric power converter has been studied (for example, see Japanese Patent Publication Sho 61-59479) (entire content of which is incorporated here by reference). With this feedwater apparatus, when one of the reactor feedwater pumps driven by the electric motors falls into a condition that water cannot be supplied according to a certain cause (trip), a backup reactor feedwater pump driven by the electric motor is started up quickly and activated for a backup by an interlock.
A quick start of the electric motor requires larger current than normal operation. On the other hand, a current which can be supplied to the electric power converter is limited, and only limited current can be supplied to the electric motor. Therefore, it takes a certain time for the electric motor to complete the start up. However, since a power supply of a capacity more than that required for a normal operation must be installed, it is not economical to install the power supply which can supply current required for the start up of the electric motor quickly.
Therefore, without a large-scale power supply, when a signal of a backup start up is transmitted, it takes typically about 10 seconds for the backup feedwater pump and the electric motor driving it to reach a minimum rotation speed from a halt condition, and takes about 15 seconds to reach a rated rotation speed.
The trip of one of the feedwater pumps causes a rapid decrease of feedwater flow rate. On the other hand, because the backup feedwater pump started up for a backup has a characteristics of slow start up, it takes a long time to reach a rated flow rate. Therefore, in spite of the installation and the start up of the backup feedwater pump, it is possible that a water level of the reactor continues falling and the nuclear reactor scrams.
As described above, with the feedwater apparatus equipped with the feedwater pump driven by a variable rotation speed electric motor using the electric power converter, it is difficult to start up the electric motor driving the backup feedwater pump quickly. Therefore, there is a problem that the feedwater flow rate to the nuclear reactor falls temporarily when one of the feedwater pumps trips.
Conventionally, a malfunction of the electric power converter among trip factors of the feedwater pump is detected by a self-diagnosis of the electric power converter. Therefore, depending on the status of the malfunction of the electric power converter, the self-diagnosis may not work normally, and the malfunction may not be detected. In this case, the backup feedwater pump of the nuclear reactor driven by the electric motor may not start up for a backup, and the feedwater flow rate may decrease, so the reactor water level also continues falling and the nuclear reactor may scram.